Many industrial processes or systems require or benefit from cooling to extremely low temperature levels. Some examples are: the cooling of cyrogenic biological storage refrigerators where liquid nitrogen is not available, and/or where safety and costs can be improved; cooling apparatus which incorporates superconductive materials; cooling high speed computers which operate more efficiently at low temperatures; and cooling detectors which need to operate at very cold temperatures, e.g. gamma-ray sensors, etc. In addition to the various applications for ultra low temperature systems, there has been a continuing need and effort to develop improved low temperature systems having greater energy efficiency while also comparing favorably with prior systems with regard to cost, availability and safety. The present invention substantially alleviates these problems.
In the field of refrigeration, high stage auto-refrigeration cascade (ARC) systems are well known, and examples thereof are described in U.S. Pat. Nos. 3,768,273; 4,597,267; and 4,763,486. An ARC system combines vapor compression and vapor-liquid adsorption refrigeration principles using a refrigerant comprising a mixture of components of varying boiling points which do not form azeotropes. Pressurized vapors are dissolved into liquid phases in an adsorption process. Typical ARC system operation is as follows: (1) A compressor raises the pressure of the mixed refrigerant, (2) a condenser partially liquifies the mixture by rejection of heat to cooling water or ambient air, (3) an economizing heat exchanger may be included to further cool and further partially condense the mixture, (4) a phase separator removes the liquid condensate from the uncondensed vapor, (5) a throttle reduces the pressure of the condensate to the suction pressure, (6) the evaporating liquid leaving the throttle is mixed with vapors returning from colder portions of the system, and (7) this cold stream removes heat in a cascade condenser from the high pressure vapors exiting from the phase separator to further or completely condense the vapors. The steps of partial condensation, phase separation and throttling of the condensate are repeated as needed to provide liquid refrigerant mixes at intermediate boiling temperatures and at very low temperatures for final cooling.
Another type of refrigeration involving the use of cryosorption pumps which operate at above atmospheric pressure has been disclosed in publications such as: "Cryogenic Refrigeration Concepts Utilizing Adsorption Pumping in Zeolites", W.H. Hartwig, Univ. of Texas, Austin, TX, 1978 published in Advances in Cryogenic Engineering, Vol. 23 (1978); "Nitrogen Adsorption Isotherms for Zeolite and Activated Carbon", L.C. Yang, T.D.Vo., and H.H. Burris, Jet Propulsion Laboratory, C.I.T., Pasadena, CA (NASA contract NAS7-100), 1982; and "High Pressure Adsorption Isotherms of Hydrogen and Neon on Charcoal", E. Tward, C. Marcus, C.K. Chan, J. Gatewood, and D. Elleman (Jet Propulsion Laboratory/C.I.T., Pasadena) and W.A. Steyert (Los Alamos National Laboratory, Los Alamos, NM. These papers detail cryosorption gas pumping at above atmospheric pressure and the adsorbed gas-to-sorbent mass ratio vs. temperature for several potentially useful gases for refrigeration effects.
Still other prior art refrigeration systems capable of producing temperatures in the same range of interest as the present invention, (e.g. 90K) includes refrigeration apparatus employing helium gas as a refrigerant and utilizing Stirling, Solvay, Gifford-McMahon cycles or variations thereof. Almost all of these prior systems require some form of lubricated or oilless reciprocating or turbine mechanical gas pumps and expanders. Modified Gifford-McMahon two-stage cryorefrigerators, with activated carbon bonded onto a portion of the colder stage, have also been used to cryopump atmospheric gases from high vacuum chambers to very deep levels. They typically operate over a pressure range from about 10.sup.-6 to 10.sup.-12 atmospheres. They employ cryocondensation for most atmospheric gases and cryosorption for the lowest boiling point gases such as hydrogen and helium. Such devices do not provide the performance and advantages of the present invention.
Helium cryorefrigerators were also found to be not readily usable for cooling of remote heat loads because their inherent configuration limits them to the cooling of surfaces attached to a body where helium gas is expanded. It is possible to provide an intermediate heat transfer mechanism, such as a heat pipe or thermo-siphon circuit, but this requires an added temperature difference and adds considerable cost and complexity to the system which reduces reliability. Also, with these systems it is possible to cool a surface exposed to atmospheric air cold enough to liquify oxygen out of the air and thereby create hazardous conditions. Most helium cryorefrigerators use oil lubricated compressors and elaborate oil separation systems. The last of several stages of oil removal generally is a carbon sorption bed which must be replaced periodically. This requires shutting down of the system and does not permit continuous operation for extended periods of several years. Many such systems use no-work expansion of the gas to produce cooling, but this is not thermodynamically efficient. Adding an expander which does work increases the complexity of the system. These systems are based upon mature technology and have proven reasonably reliable, although are costly because of the special components they require.
The high vacuum industry uses sorption "roughing" pumps, usually with zeolites such as "molecular sieves", to provide clean pumping prior to switching to deep vacuum pumping by other types of pumps such as titanium sublimation, ion capture, or helium refrigerated cryopumps. These sorption roughing pumps remove gases from vacuum chambers over a range starting at about 0.1 atmosphere and pumping to about 10.sup.-6 atmosphere, that is, a span of about five decades in the sub-atmospheric pressure range. They are cooled by liquid nitrogen to a temperature of about 77-80K for the cryosorption process and electrically heated to 450-500K for regeneration or desorption. The external electric heat source must be temperature controlled to prevent overheating. These pumps cannot be used at pressures above one atmosphere because of their design and they are unsuitable in their present configuration for combination with mechanical refrigeration cooling or incorporation as required by the present invention.
The foregoing review of existing and prior low temperature refrigeration systems indicates the need for such an improved system that is effective and reliable as well as relatively easy and economical to operate.
It is therefore an object of the invention to provide a refrigeration system that will overcome the deficiencies and solve the aforesaid problems of prior art systems and also produce colder temperatures than heretofore possible using auto-refrigeration cascade (ARC) systems.
Another object of the invention is to provide a refrigeration system that will: (1) utilize ARC techniques in combination with (2) cryosorption pumping to reliably pump a lower stage refrigerant gas with maintenance-free operation to cool refrigerant evaporators of various configurations. Cryosorption pumping at above atmospheric pressures and at a 15:1 to 25:1 pressure ratio eliminates problems usually attendant with the pumping of gases such as nitrogen or argon. Such problems include: Very high discharge temperatures at such high pressure ratios, lubrication of compressors or seals, maintenance of oilless compressors if used, and complexity of multi-stage compression.
Yet another object of the invention is to provide a refrigeration system which does not require extraneous temperature controls, or other devices which add complexity to the system for functions such as controlling the temperature of the sorbent beds during desorption or of the final evaporator to preclude oxygen condensation.